It must be noted that in MPEG the order of the picture in the coded stream
is the order in which the decoder process them. The reconstructed frames are
not necessarily in the correct form of display. The following example in
table 4.7 shows
such a case at the encoder output, in the coded bitstream and at the decoder
input, and at the decoder output.

Table 4.7:
MPEG Source, Encoder and Decoder Frame Sequence

At the encoder input

12

3

4

5

6

78

9

10

11

12

13

IB

B

P

B

B

PB

B

I

B

B

P

At the encoder output

14

2

3

7

5

610

8

9

13

11

12

IP

B

B

P

B

BI

B

B

P

B

B

At the decoder output

12

3

4

5

6

78

9

10

11

12

13

Since the order of pictures at the decoder is not always in
the display order, this leads to potential for delays in the
encoder/decoder loop
This is also true of H.261 - at its highest compression ratio, it may
incur as much as 0.5 seconds delay - not very pleasant for interactive
use!